US 20040179625 A1 Abstract An apparatus and method for performing coarse frequency synchronization in an orthogonal frequency division multiplexing (OFDM) receiver are provided. The coarse frequency synchronization apparatus in a frequency synchronizer of an orthogonal frequency division multiplexing (OFDM) receiver includes a buffer that receives a demodulated symbol and outputs a shifted symbol generated by cyclically shifting the symbol by a predetermined shift amount; a controller that determines the length of an summation interval according to a phase coherence bandwidth and a number of sub-bands into which the summation interval is divided, and generates and adjusts a symbol time offset according to the number of sub-bands; a reference symbol predistortion portion that generates a reference symbol whose phase is distorted by the symbol time offset; a counter that counts the shift amount; a partial correlation portion that receives the shifted symbol and the reference symbol and calculates a partial correlation value for each of the sub-bands; and a maximum value detector that calculates the shift amount where the partial correlation value is a maximum and outputs the same as an estimated coarse frequency offset value.
Claims(15) 1. A coarse frequency synchronization apparatus in a frequency synchronizer of an orthogonal frequency division multiplexing (OFDM) receiver, the apparatus comprising:
a buffer operable to receive a demodulated symbol and output a shifted symbol generated by cyclically shifting the demodulated symbol by a predetermined shift amount; a controller operable to determine a length of summation interval according to a phase coherence bandwidth and a number of sub-bands into which the summation interval is divided, and generate and adjust a symbol time offset according to the number of sub-bands; a reference symbol predistortion portion operable to generate a reference symbol whose phase is distorted by the symbol time offset; a counter operable to determine the shift amount; a partial correlation portion operable to receive the shifted symbol and the reference symbol and calculate a partial correlation value for each of the sub-bands; and a maximum value detector operable to calculate the shift amount where the sum of the partial correlation values is a maximum and output the shift amount as an estimated coarse frequency offset value. 2. The apparatus of where X(k+d) represents the shifted demodulated symbol, Z(k) represents the reference symbol, N is a number of subcarriers, K is the number od sub-bands and d is the predetermined shift amount and is a value between
3. The apparatus of a reference symbol generator operable to generate a phase reference symbol; and a phase rotation portion operable to rotate the phase of the phase reference symbol according to the symbol time offset value and output a phase-distorted reference symbol. 4. The apparatus of 5. The apparatus of _{off }where T_{off }is a maximum time offset for which frame synchronization can be achieved. 6. A coarse frequency synchronization method for use in an orthogonal frequency division multiplexing (OFDM) receiver for performing OFDM demodulation and frequency synchronization, the method comprising:
(a) receiving a demodulated symbol and outputting a shifted symbol generated by cyclically shifting the symbol by a predetermined shift amount; (b) determining the length of a summation interval according to a phase coherence bandwidth and a number of sub-bands into which the summation interval is divided, and generating a predetermined symbol time offset according to the number of sub-bands; (c) generating a reference symbol whose phase is distorted by the symbol time offset; (d) counting the shift amount; (e) calculating a partial correlation value between the shifted symbol and the reference symbol for each of the sub-bands; and (f) determining the shift amount d where the partial correlation value is a maximum and outputting the shift amount d as an estimated coarse frequency offset value. 7. The method of where X(k+d) represents the shifted demodulated symbol, Z(k) represents the reference symbol, N is a number of subcarriers, K is the number of sub-bands and the predetermined shift amount d is a value between
8. The method of (c 1) generating a phase reference symbol; and (c 2) rotating the phase of the phase reference symbol according to the symbol time offset value and outputting a phase-distorted reference symbol. 9. The method of 2), a complex number corresponding to each of a plurality of subcarriers, by which a phase is rotated, is generated, and the generated complex number is multiplied by the phase reference symbol to generate a phase-distorted reference symbol. 10. The method of _{off }where T_{off }is a maximum time offset for which frame synchronization can be achieved. 11. An orthogonal frequency division multiplexing (OFDM) receiver including a coarse frequency synchronization apparatus, the apparatus comprising:
a buffer that receives a demodulated symbol and outputs a shifted symbol generated by cyclically shifting the symbol by a predetermined shift amount; a controller than determines the length of a summation interval according to a phase coherence bandwidth and a number of sub-bands into which the summation interval is divided, and generates and adjusts a symbol time offset according to the number of sub-bands; a reference symbol predistortion portion that generates a reference symbol whose phase is distorted by the symbol time offset; a counter that counts the shift amount; a partial correlation portion that receives the shifted symbol and the reference symbol and calculates a partial correlation value for each of the sub-bands; and a maximum value detector that calculates the shift amount d where the partial correlation value is a maximum and outputs the shift amount d as an estimated coarse frequency offset value. 12. The receiver of where X(k+d) represents the shifted demodulated symbol, Z(k) represents the reference symbol, N is a number of subcarriers, K is the number of sub-bands and the predetermined shift amount d is a value between
13. The receiver of a reference symbol generator that generates a phase reference symbol; and a phase rotation portion that rotates the phase of the phase reference symbol according to the symbol time offset value and outputs a phase-distorted reference symbol. 14. The receiver of 15. The receiver of _{off }where T_{off }is a maximum time offset for which frame synchronization can be achieved.Description [0030] To aid in understanding a coarse frequency apparatus and method according to the present invention, a correlation value and a phase coherence bandwidth applied in accordance with this invention will now be described. [0031] First, to identify the effect of carrier frequency offset of a reception signal, it is assumed that the k-th subcarrrier reception frequency of the reception signal is f [0032] where Δf denotes a number that represents the frequency offset of subcarrier by a multiple of subcarrier spacing and can also be replaced by the sum of an integer Δf [0033] where C [0034] Meanwhile, if the frequency offset does not contain an error represented by an integer multiple, i.e., Δf [0035] The demodulated signal {overscore (C)} [0036] This means the frequency error represented by an integer multiple of frequency error causes a signal intended for demodulation to be shifted by −Δf [0037] Thus, in a coarse frequency synchronization method according to this invention, a correlation value is calculated by sequentially rotating the already known phase reference symbol and reception signal by symbol intervals, and the amount of rotation where the maximum correlation value occurs is determined as an integer multiple of frequency error. This relationship is defined by Equation (8): [0038] where ((k+d)) [0039] The phase coherence bandwidth for the reception signal and phase reference signal in a digital audio broadcasting (DAB) system using OFDM will now be described. In general, channel coherence bandwidth refers to a statistically measured frequency band where a channel can be deemed as ‘flat’ or passes two signals so that they have approximately the same gain and linear phase over all spectrum components. That is, a channel coherence band is a frequency band in which two different frequency components have a strong correlation. In this case, assuming that the coherence band of a channel is B [0040] A phase coherence bandwidth is defined as a frequency interval where two signals having a delay in the time domain and generated by performing a DCT on the same signal maintain their correlation in the frequency domain. Analogous to the channel coherence band, it can also mean a frequency band in which the two signals have a strong correlation. [0041] Let a time domain signal in the OFDM system be z(t), a delayed signal having a frequency error of T [0042] Here, for convenience in expanding the equation, both noise and frequency error are neglected, and N denotes the number of subcarriers. [0043] As described above, the phase coherence bandwidth is defined as a frequency band in which two signals always have a strong correlation. That is, if a frequency band B has the largest bandwidth where a correlation value of the two signals Z(k) and e [0044] where T [0045] These requirements are applicable to a DAB system. Since in Equation (11) the lower bound m of the summations is not fixed, the relation between the frame synchronization error T [0046] Furthermore, Equation (12) can be combined with Equation (10) to yield Equation (13) which can be used to obtain a phase coherence bandwidth with respect to changes in a frame synchronization error: [0047] The left side of Equation (13) is a correlation function of two signals z(t) and Z(t+T [0048]FIG. 2 is a graph showing computer simulation results of the relation in Equation (13). The graph illustrates the relationship between phase coherence bandwidth and time offset between an original signal and a signal having a frame synchronization error with respect to the original signal. Referring to FIG. 2, a bandwidth is represented by a multiple of subcarrier spacing, and the entire frequency band of a channel is set to [0049] To reveal the fact that there is a reciprocal relationship between a time delay factor and coherence bandwidth, FIG. 2 shows the relationship between coherence bandwidth and a time offset T [0050] Meanwhile, it is assumed that z(t) and x(t) are a reference signal and a reception signal generated by performing IFFT on the phase reference signal Z(k) and signal X(k) in Equation (8), respectively. It is further assumed that the reception signal x(t) has a time delay, i.e., frame synchronization error. Given these assumptions, there is a reciprocal relation between a frame synchronization error Δt and phase coherence bandwidth on the frequency axis. This relationship means that as the frame synchronization error Δt increases, phase coherence bandwidth on the frequency axis decreases. [0051] The coarse frequency synchronization method of the present invention is based on coarse frequency synchronization using a correlation value between reference signals. Referring to FIG. 5, when calculating a correlation value according to the present invention, a summation interval BWLen is set to be smaller than a phase coherence bandwidth calculated for a reference symbol and a reception symbol having a time offset. That is, in order to calculate a correlation value between a shifted reception symbol and a reference symbol, the summation interval is divided into a plurality of intervals that are smaller than a phase correlation bandwidth of two signals, partial correlation values are calculated for each small interval resulting from the division, and an average or sum of the partial correlation values is taken to determine a shift amount where the maximum correlation value is generated. [0052] Since this method excludes a decorrelation band where accurate frame synchronization between reference and reception symbols is not achieved, the correlation function value is always meaningful. Thus, coarse frequency synchronization is accurately performed within the range of a time offset that can be tolerated in frame synchronization. These principles are applied to the coarse frequency synchronization apparatus and method of the present invention. [0053] Furthermore, in order to estimate a coarse frequency offset, the present invention generates a predistorted phase reference. First, the effect of symbol distortion with respect to a time offset will now be described with reference to FIG. 6. [0054] As shown in FIG. 6, a time offset in OFDM causes phase rotation in proportion to the order of a subcarrier in a sequence of subcarriers. For mathematical convenience, it is assumed that only one symbol interval of the entire signal is used, a transmission channel is an Additive White Gaussian Noise (AWGN) channel, and accurate frequency synchronization in a reception signal is achieved. If time synchronization does not occur in an OFDM system, a reception signal r [0055] where N is the number of subchannels, C [0056] where f [0057] Here, in order to observe the effect of time synchronization on the reception signal, it is assumed that sampling is performed with a period [0058] In this case, T [0059] While satisfying the above conditions, a discrete signal generated by sampling the reception signal expressed in Equation (15) is given by Equation (16): [0060] A demodulated signal {overscore (C)}′ [0061] Here [0062] is expressed by Equation (18): [0063] Here, a in Equation (18) is given by Equation (19): [0064] That is, since k, p, and N are all integers in Equation (19), k−p needs to be an integer multiple of N such that a=1. Otherwise, a [0065] Since it is possible to apply the conditions of Equation (20) to Equation (17) only if α=0, the result is expressed by Equation (21): [0066] Furthermore, the term P [0067] Therefore, it is evident from Equation (21) that due to the effect of an error occurring when time synchronization is not achieved, the reception signal {overscore (C)}′ [0068] That is, the reception signal in Equation (21) suffers phase rotation due to a time offset. The phase is rotated by τ*p in proportion to the order p of subcarriers. Thus, the present invention generates a reference symbol whose phase has been predistorted and uses the reference symbol in detecting coarse frequency synchronization, thus allowing more accurate frequency synchronization detection. Here, τ corresponds to a symbol time offset Δt [0069] A coarse frequency synchronization apparatus and method according to a preferred embodiment of the present invention will now be described with references to FIGS. 3A and 4. FIG. 3A is a block diagram showing an example of the structure of an OFDM receiver having the coarse frequency synchronization apparatus according to an embodiment of the invention. The OFDM receiver shown in FIG. 3A is comprised of an OFDM demodulator [0070] Since the OFDM demodulator [0071] In step S [0072] In step S [0073] In step S [0074] In step S [0075] the partial correlation portion [0076] of partial correlation values. [0077] In step S [0078] According to the present invention, an algorithm for calculating Equation (22) is repeated a number of times predetermined by the controller [0079] where N is the number of subcarriers, K is the number of sub-intervals into which the interval of summation with respect to a correlation function is divided, and N/K is a sub-interval BWLen (See FIG. 5) of the summation. Thus, assuming that an individual sub-band width is BW [0080] The configuration and operation of the reference symbol predistortion portion [0081] The process of predistorting the reference symbol will now be described. The controller [0082] The phase rotation portion [0083] Although it has been described that the phase rotation portion [0084]FIGS. 7A and 7B show correlation between the reference signal and the reception signal with respect to change in symbol time offset. It is evident from these figures that coherence bandwidth varies with symbol time offset. Thus, by performing a coarse frequency synchronization algorithm after setting an appropriate coherence bandwidth, a coarse frequency offset can be effectively estimated. [0085] To check if a coarse frequency offset value can be properly detected by the coarse frequency synchronization apparatus and method of this invention, a simulation was performed. FIGS. 8A and 8B are graphs illustrating the results of this simulation. For the purpose of comparison, FIGS. 9A and 9B show the results of a simulation performed according to a conventional coarse frequency offset value detection method. [0086] The simulations were performed under the conditions that signal-to-noise (SNR) ratio is 5 dB in a Gaussian channel, the number of subcarriers is [0087] Referring to FIGS. 8A and 8B, the results of the simulation performed according to the coarse frequency synchronization method of the present invention show that a maximum value occurs at the value of −62 on the horizontal axis, corresponding to the given frequency offset value in the case of no time offset as shown in FIG. 8A as well as in the case of a time offset of 10.0 as shown in FIG. 8B. This shows that the coarse frequency offset value is accurately detected in the present invention. [0088] In contrast, referring to FIGS. 9A and 9B, in the conventional coarse frequency offset value detection method, the coarse frequency offset value is accurately detected in the case of no symbol time offset as shown in FIG. 9A. However, in the case of a symbol time offset of 10.0, as shown in FIG. 9B, no peak appears at the value of −62 on the horizontal axis corresponding to the given frequency offset value, showing that the coarse frequency offset value was not accurately detected. [0089]FIGS. 10A-10D are graphs illustrating percentage probability of accurately detecting a course frequency offset value versus frame synchronization error value. The purpose of these graphs is to explain accuracy in error detection using the coarse frequency synchronization method of the present invention. Here, both theoretical data and data obtained by simulation are plotted. The simulation conditions are that the channel is a Gaussian channel with an SNR ratio of 5 dB and a sample time offset interval is in the range of −50-+50. The frequency offset is a value in the range of −510 to +510. [0090]FIGS. 10A-10D show for comparison theoretical intervals and intervals obtained by simulations where a frequency offset value can be accurately detected according to the coarse frequency synchronization method of the present invention. The theoretical intervals are represented by a thick solid line and the intervals obtained by simulation are represented by a thin solid line. In the simulations, the number of subcarriers is [0091] Referring to FIGS. 10A-10D, where the width BW [0092] According to the method of the present invention, the amount of computation is proportional to N [0093] Thus, the method of the present invention can reduce the additional amount of computation [0094] within the tolerance given by a coarse frame synchronization algorithm while maintaining stable operation. The exact amount by which computation is reduced varies depending on the number of subcarriers. Specifically, the amount by which computation is reduced is the same as the amount of computation required to perform N IFFT operations where N is the number of subcarriers. For example, if the number of subcarriers is [0095] While this invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Thus, the scope of the present invention is limited not by the foregoing but by the following claims, and all differences within the range of equivalents thereof should be interpreted as being within the scope of the present invention. [0096] As described above, the coarse frequency synchronization method and apparatus according to the present invention can prevent degradation in performance when a symbol timing error is greater than or equal to ±1(±½) by dividing the summation interval for calculating a correlation value between the received symbol and reference symbol into a predetermined number of sub-intervals determined according to a local coherence bandwidth. [0097] In particular, this invention can accurately achieve coarse frequency synchronization even when a symbol timing error is greater than ±5 samples under poor channel conditions, by predistorting the reference symbol used in calculating a correlation value with the received symbol. [0019] The above objects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: [0020]FIG. 1 shows an example of the structure of a conventional orthogonal frequency division multiplexing (OFDM) receiver; [0021]FIG. 2 is a graph illustrating the relationship between phase coherence bandwidth and time offset between an original signal and a delayed signal having a frame synchronization error with respect to the original signal, in order to explain a coarse frequency synchronization apparatus and method according to the present invention; [0022]FIG. 3A is a block diagram showing the structure of an OFDM receiver having a coarse frequency synchronization apparatus according to an embodiment of the present invention, and FIG. 3B is a detailed block diagram showing the structure of a reference symbol predistortion portion shown in FIG. 3A; [0023]FIG. 4 is a flowchart illustrating a coarse frequency synchronization method according to an embodiment of the present invention; [0024]FIG. 5 schematically shows a process of calculating the correlation between a received reference symbol and a predistorted reference symbol according to the present invention; [0025]FIG. 6 explains the effect of symbol distortion with respect to a time frequency error; [0026]FIGS. 7A and 7B show correlation between a reference symbol and a reception symbol with respect to changes in a symbol time offset; [0027]FIGS. 8A and 8B show results of a simulation of a coarse frequency offset value detection method of the present invention; [0028]FIGS. 9A and 9B show results of a simulation of a conventional coarse frequency offset value detection method; and [0029]FIGS. 10A-10D are graphs showing both theoretical and simulated percentage probabilities of accurately detecting a frequency offset value using a coarse frequency synchronization method according to the present invention versus frame synchronization error. [0001] This application claims priority from Korean Patent Application No. 2003-16287, filed Mar. 15, 2003, the contents of which are incorporated herein by reference in their entirety. [0002] 1. Field of the Invention [0003] The present invention relates to a coarse frequency synchronization method and apparatus for an orthogonal frequency division multiplexing (OFDM)—based system, and more particularly, to a coarse frequency synchronization method and apparatus in an OFDM receiver. [0004] 2. Description of the Related Art [0005]FIG. 1 is a block diagram showing the structure of a conventional OFDM receiver system. Referring to FIG. 1, the conventional OFDM receiver system includes an OFDM demodulator [0006] The operation of the conventional OFDM receiver system of FIG. 1 will now be described. First, when the RF receiver [0007] The demodulated signal is stored in the buffer register [0008] In this case, the reception signal X [0009] The result of Equation (2) is obtained using the process for calculating convolutions of two signals in the time domain, and the resulting value h [0010] Using this relationship, a maximum value detector [0011] where Z denotes a phase reference symbol and X [0012] A coarse frequency synchronization method according to the conventional OFDM receiver system discussed above makes it possible to actually or theoretically estimate a frequency error in any situation regardless of channel environment and frame synchronization error. However, this method requires a considerable amount of computation. Specifically, this method requires a very complicated IFFT module in order to accurately estimate a frequency error, and causes an excessive time delay due to a long response time. [0013] The present invention provides a coarse frequency synchronization apparatus in an orthogonal frequency division multiplexing (OFDM) receiver capable of performing stable frequency synchronization with a small amount of computation. [0014] The present invention also provides a coarse frequency synchronization method implemented in the coarse frequency synchronization apparatus. [0015] The present invention also provides an orthogonal frequency division multiplexing receiver that can perform stable synchronization with a small amount of computation. [0016] According to an aspect of the present invention, there is provided a coarse frequency synchronization apparatus in a frequency synchronizer of an orthogonal frequency division multiplexing (OFDM) receiver. The coarse frequency synchronization apparatus includes: a buffer operable to receive a demodulated symbol and output a shifted symbol generated by cyclically shifting the demodulated symbol by a predetermined shift amount; a controller operable to determine a length of summation interval according to a phase coherence bandwidth and a number of sub-bands into which the summation interval is divided, and generate and adjust a symbol time offset according to the number of sub-bands; a reference symbol predistortion portion operable to generate a reference symbol whose phase is distorted by the symbol time offset; a counter operable to determine the shift amount; a partial correlation portion operable to receive the shifted symbol and the reference symbol and calculate a partial correlation value for each of the sub-bands; and a maximum value detector operable to calculate the shift amount where the sum of the partial correlation values is a maximum and output the shift amount as an estimated coarse frequency offset value. [0017] According to another aspect of the present invention, there is provided a coarse frequency synchronization method for use in an OFDM receiver for performing OFDM demodulation and frequency synchronization. The method includes the steps of: (a) receiving a demodulated symbol and outputting a shifted symbol generated by cyclically shifting the symbol by a predetermined shift amount; (b) determining the length of a summation interval according to a phase coherence bandwidth and a number of sub-bands into which the summation interval is divided, and generating a predetermined symbol time offset according to the number of sub-bands; (c) generating a reference symbol whose phase is distorted by the symbol time offset; (d) counting the shift amount; (e) calculating a partial correlation value between the shifted symbol and the reference symbol for each of the sub-bands; and (f) determining the shift amount d where the partial correlation value is a maximum and outputting the shift amount d as an estimated coarse frequency offset value. [0018] According to yet another aspect of the present invention, there is provided an OFDM receiver including a coarse frequency synchronization apparatus. The coarse frequency synchronization apparatus is comprised of: a buffer that receives a demodulated symbol and outputs a shifted symbol generated by cyclically shifting the symbol by a predetermined shift amount; a controller than determines the length of a summation interval according to a phase coherence bandwidth and a number of sub-bands into which the summation interval is divided, and generates and adjusts a symbol time offset according to the number of sub-bands; a reference symbol predistortion portion that generates a reference symbol whose phase is distorted by the symbol time offset; a counter that counts the shift amount; a partial correlation portion that receives the shifted symbol and the reference symbol and calculates a partial correlation value for each of the sub-bands; and a maximum value detector that calculates the shift amount d where the partial correlation value is a maximum and outputs the shift amount d as an estimated coarse frequency offset value. Referenced by
Classifications
Legal Events
Rotate |